Shock tubes and signal tubes are classes of low-energy fuse used in blasting systems for transmitting an initiation signal from one point to another (usually from one detonator or pyrotechnic delay to another), such tubes being constructed of plastic, usually extruded and unreinforced, and containing a particular detonating or rapid reacting pyrotechnic composition distributed substantially uniformly along its central core at relatively low loadings compared to common detonating cords. The particulate composition is loosely adherent to the inner wall of the tube so that it is shock-dislodgeable. The internal bore of the tubing is usually narrow, and is normally circular (though it need not be). Shock tube, for example, will typically consist of extruded plastic tube of internal diameter around 1 mm with a core loading of, say, HMX/AL (94:6 parts by weight) of below 20 mg/m. Signal tube designed for lower signal transmission speeds (i.e. significantly below 2 km/s) will have similar dimensions, and will contain a rapid reacting pyrotechnic composition comprising a metal fuel e.g. Al or quasi-metal fuel such as Si and a powerful inorganic oxidising agent (especially BaO.sub.2) typically at a core loading of around 20 mg/m to 100 mg/m. Reference may be made to European Patent No. 327 219 (ICI) for further information on shock tube products.
In field or mine situations it is not always immediately apparent to a blast engineer that a particular tube has fired merely from visual inspection of the still intact tube. This is in part because the visible colour change of the core material upon detonation or reaction may not be significant, especially at low core loadings. A further reason is that accessories producers prefer to supply coloured products and so the plastic of the shock/signal tube usually will be self-coloured, thus masking to a significant degree any core colour change that might otherwise have been perceptible. Additionally, natural or artificial light levels, especially underground, are not always at an intensity or spectral breadth conducive to perceiving a colour change in core material.
The Applicants have experimented with adding reactive pigment particles to the shock/signal tube core charge. The results were generally poor because, for a noticeable colour change, levels of pigment had to be used which caused fundamental disturbance of the firing performance of the tube. The present invention has overcome this problem, allowing achievement of marked colour change while using only a relatively small amount of reactive pigment.